In modern wells, typically used for the exploitation of underground fluid reserves, a tubular bore lining, known as a completion, must be installed to support the wellbore throughout the life of the well. The completion may be required to allow controlled flow of reserves from several discrete reservoir sections.
Following drilling of a wellbore through a sandstone reservoir, it is often a requirement that the borehole be completed with a device that retains the sand particles in the reservoir, yet allows the hydrocarbons or water to be produced to surface with a generally low solids content. Several methods exist for “sand control”. Such methods have been continuously developed since commercial exploitation of underground hydrocarbon resources began over 100 years ago.
At present in the energy and water industries, the accepted best practice is to install a sand control device that provides support to the wellbore face. Perhaps the oldest technique for providing support to the wellbore face is the placement of loose gravel around a rigid sand screen filter, otherwise known as gravel-packing (GP). If placed correctly, the gravel can completely fill the annular void between the screen and the borehole wall, maximizing support.
More recently devices have been developed to provide wellbore support without the need to pump gravel between the screen and the wellbore face. So-called expandable completions (EXP) rely on the plastic yielding of a tubular member to increase its diameter therefore minimizing or eliminating the annular void.
Both GP and EXP completions are operationally intensive activities. In the case of GP, several thousand barrels of specialized completion fluids and hundreds of tonnes of gravel must be prepared and pumped downhole to fill the void in a modern horizontal well. Such wells may exceed 4000 ft of reservoir penetration, traversing several rock types and of infinitely varying properties. If the operation is interrupted due to an equipment failure at surface, or because the rock characteristics are different to those assumed, the entire job could fail, resulting at best in a sub-optimal completion and at worst, with the well being lost. The equipment required to pump large GP treatments in modern wells requires capitally intensive investment. In the case of remote offshore wells, dedicated boats may be required to be built to support the operation. Tens of service personnel may be required to effect a GP installation. Accordingly, this is expensive and in times of high activity may result in jobs being postponed until enough skilled labour is available. It is not uncommon for GP treatments in horizontal wells to cost several million US dollars per well.
In addition to sand control requirements, reservoirs may need to be divided up into discrete pressure containing zones. In this case the completion must facilitate the isolation of one zone from another with a potential differential pressure across zones. Such isolation becomes difficult when it must be combined with sand control. This is especially the case with GP and is one driver for the development of EXP completions with integral zonal isolation. Zonal isolation takes many forms: open hole, between casings or behind casing and achieving isolation correctly and economically is still an important aspect of well design. More recently, swelling elastomers have been developed as an oil-field method of achieving zonal isolation.